Indigo and Azure are the Blue team. Maroon and Scarlet are the Red
team.

In negotiating the barriers, knights are assumed to move first orthogonally.
You may capture a unit from an allied army, but you do not check
the allied king.
You may not move into check. If a king is checkmated when his turn
comes around, his whole army vanishes. If there is no legal move, skip
the turn. For this reason, stalemate is unlikely, so all pawns promote
to queens.

I couldn't figure out how to handle the en passant rule,
so I skipped it.
I may add the option to arrange the pieces in your fortress
as you like at the onset.

I figured the pawns could capture around the corner of the fortress,
but that bishops could not sneak out the rim of the door.